1. Field of the Invention
This invention relates to electronic circuits and, more particularly, to programming non-volatile programmable fuses and endpoint detection.
2. Description of the Related Art
Mixed-signal and analog integrated circuits (ICs) may require post-fabrication trimming or calibration to center reference voltages, set oscillator frequency, or adjust other critical parameters within the chip to conform to specifications and meet customer requirements. Several methods exist for trimming ICs, such as programming metal or polysilicon fuses, zapping zener diodes, or programming EPROMs and EEPROMs.
Fuses are two-terminal devices that may be used for trimming ICs. Usually when unprogrammed, a fuse is a low resistive element; but the fuse becomes a high resistive element after “blowing” or programming the fuse to trim a particular IC. In some cases, programming a fuse results in an open circuit. Fuses are typically non-volatile, one-time programmable elements made from materials such as tungsten, nichrome, or polysilicon. Metal fuses usually require larger programming currents than polysilicon fuses; therefore, metal fuses may not be practical for some devices that use thin gate oxides. In these devices, polysilicon fuses or multi-layer polysilicon fuses having a silicide layer may be a solution for the trimming or calibration requirements.
In some applications, silicide polysilicon fuses may be used as on-chip, non-volatile programmable elements. Silicide fuses comprise a polysilicon fuse body and generally a thin layer of titanium silicide (TiSi2), which has a lower resistance than the polysilicon fuse body. Other types of silicide layers may be used, such as tungsten silicide, tantalum silicide, or platinum silicide. Programming of a silicide fuse may be achieved by forcing a current through the fuse to cause the silicide top layer to heat up and eventually melt and agglomerate with the underlying polysilicon fuse body. After programming, the silicide fuse has an increased resistance since the agglomeration of the low resistance silicide layer with the polysilicon body results in a discontinuity in the fuse.
Some fuses may end up completely open after programming, which means that the polysilicon body is completely interrupted. If this occurs, it is possible that structures in the vicinity of the fuse may have been damaged due to the programming of the fuse. For example, cracks in the oxide layers may commonly occur in such instances. This type of damage may be incurred as a result of the various techniques currently used to program fuses. For example, many such techniques over-program the fuse elements.
One mechanical method to overcome this problem and prevent damage to the fuse and surrounding structures from emitted material is to include passivation openings on top of the fuse element. Passivation openings are intended to allow hot material to escape, thereby hopefully preventing damage underneath the fuse. Another mechanical method that may prevent unwanted damage is placing seal ring structures around the fuse. The seal ring structures can be a combination of stacked metal layers to create a wall around a fuse. Both solutions are often combined but these mechanical techniques do not offer a reliable solution to prevent unwanted damage from over-programming fuse elements.